1. Field of the Invention
The invention relates to the improvement of the resistance to wear and cavitation exhibited by plastics overlays on composite materials used as bearings for oil-lubricated applications. In particular, the invention provides an increase in the service life of shock absorbers and composite bearings used under shock absorber-type conditions.
2. Description of Related Art
Materials used for composite bearings with plastics overlays consist of a metallic backing layer, preferably of steel, bronze or a high-strength aluminum alloy, and a sealed plastics overlay applied directly to the metal backing. As an alternative to this, the plastics overlay may also be applied to a porous intermediate layer of sintered metal, in such a way that the pores are also completely filled with plastics material. The plastics overlay comprises PTFE as its base material and additionally contains wear- and friction-reducing additives.
For bearing systems such as these where oil is present, a very low coefficient of friction accompanied at the same time by a high degree of resistance to wear and cavitation is very important, for example in shock absorbers which are used in the automobile industry or in gear pumps or hydraulic motors. Owing to the low coefficients of friction necessary, current knowledge insists that such materials consist to a considerable extent of PTFE, since no other known plastics satisfy these requirements, even when their other properties are suitable.
Since PTFE alone is too soft, and therefore has a high coefficient of wear, materials must be mixed with it which counter cavitation and wear without substantially increasing the coefficient of friction.
The bearing materials used most commonly at present in the above-mentioned area of application consist of a steel backing, a porous bronze layer and a plastics material of which approximately 80 vol. % is PTFE and the rest is lead or molybdenum disulphide. The plastics material is pressed into the bronze framework and forms a thin sealed layer thereover. However, materials with this structure have a tendency to produce erosion and cavitation phenomena under extreme conditions, such as arise in shock absorbers for example through heavy loads, high sliding speeds and acceleration rates and high flow speeds in the bearing gap, and therefore have only a limited service life, especially when highly stressed.
Although it is possible to produce materials which exhibit hardly any wear and no susceptibility to cavitation under the above-described conditions if a thermoplastic material other than PTFE is used, for instance PVDF or PEEK, a marked increase in friction in the case of oil lubrication has in such cases to be accepted.
Many documents propose PTFE-based compositions which, however, have hitherto provided relatively only slight improvements over the above-mentioned standard materials and generally have an increased coefficient of friction, as in the case of calcium fluoride, put forward in EP 183375 A2, or of polyimides, as proposed in DE 4227909 C2.
WO 95/02772 proposes the use of an aramide fibre pulp to reduce wear and cavitation, but the homogeneous mixing-in of these fibres causes problems and requires appropriate additional production equipment.
In contrast thereto, it is the object of this invention to make it possible, without altering production processes and without impairment of the coefficient of friction when oil is used, to achieve service lives which surpass several times those of the corresponding materials according to the prior art.